1. Field of the Invention
A method for geophysical exploration using reflected converted shear waves derived from a compressional wave surface source. In a multilayer lithologic environment, the polarization angle of a target layer is defined by removing the birefringent effects of the overlying layers.
2. Discussion of Related Art
Seismic exploration is conducted primarily to provide petrophysical information about subsurface earth formations for the purpose of optimizing the location and configuration of boreholes used in the recovery of hydrocarbon products. The borehole shaft is configured to maximize fluid drainage from the vugs and crevices of the surrounding porous rocks. In the presence of vertical fracturing, the borehole may be deviated from a substantially vertical to a substantially horizontal axis. The specific seismic exploration approach to be employed in an area of interest is preferably tailored to display the particular geological conditions there existing, as will next be explained.
In the art of seismic exploration, a seismic source at or near the surface of the earth, radiates an acoustic wavefield that expands downwardly to insonify the elastic media comprising the rock layers beneath the surface. The seismic wavefield is reflected from the respective acoustic impedance interfaces between layers as a wavefront that propagates upwardly to an array of spaced-apart seismic receivers deployed at or near the surface of the earth, offset from the source by some desired spatial interval. The receivers detect the mechanical earth motions (or the pressure field variations in marine operations) due to the reflected seismic wavefield and convert the detected motions to electrical-signal amplitudes as a function of reflection travel time. The electrical time-scale signals are stored and processed, preferably in a programmed computer, to provide a model of the attitude and structure of the subsurface formations.
Seismic surveys may be 2-dimensional wherein a source or series of sources insonifies linear arrays of receivers along a single line of survey. In 3-D operations, the sources and receivers are distributed areally in a regular grid pattern over many square kilometers.
The source and receivers may be designed to be responsive to compressional (P) waves or shear (S) waves. P waves are longitudinal waves that propagate with particle motion perpendicular to the wavefronts as alternate compressional and rarefaction waves. P-wave receivers are responsive to vertical particle motion with respect to the surface of the earth.
Shear waves are polarized parallel to the wavefronts and are classified as SH waves and as SV waves for isotopic media. In the context of this disclosure, for SH waves, particle motion is horizontal in a plane that is transverse with respect to the line of profile. The particle motion for SV waves is in a vertical plane that is perpendicular to the SH particle motion and parallel to the line of profile. Shear waves cannot propagate in a fluid because fluids have no shear strength. Some media are birefringent to S-waves by reason of being anisotropic. That is, the acoustic energy splits into ordinary and an extraordinary ray paths characterized by different propagation velocities as fast (S.sub.f) and slow (S.sub.s) waves) during transit through a medium and with different polarization directions than pure SH or SV waves.
P-wave sources excite the earth along substantially vertical trajectories relative to the surface of the earth. S-wave sources are polarized to shake the earth horizontally. Either two such S-wave sources, orthogonally polarized, may be used at the same location or a single source may be used to first generate SH waves and then physically rotated 90.degree. to generate SV waves.
P-wave exploration comprises the workhorse of seismic surveys. But special studies, that require additional exploration of the anisotropic characteristics of selected rock formations due to stress and fracturing, may be undertaken by combining S-wave and P-wave technology in a single survey effort. However, implementation of such a combined survey would require use of three separate sources, viz.: P-wave, SH-wave and SV-wave sources, at each source station and multicomponent receivers that incorporate both P-wave and S-wave seismic receiver units at each receiver station. The need for three separate sources triples the survey costs prohibitively.
It is known that a compressional wavefront, impinging on an acoustic impedance interface, will generate not only a reflected and a refracted P-wave, but also an upward-traveling reflected converted (PS) shear wave. Thus, a single P-wave source in combination with a multicomponent receiver array, could be economically employed for a combined P-wave, S-wave survey using converted PS (compressional/shear) shear waves in place of pure SS (shear/shear) shear waves.
In U.S. Pat. No. 4,736,349, issued Apr. 5, 1988, Neal R. Goins teaches a method for obtaining shear wave data from common-depth-point-gathered compressional wave traces, corrected for angularity and spherical spreading, using variations in the amplitude of the gathered compressional waves as a function of source-receiver offset. That is said to be possible because the amplitude of embedded reflected shear wave energy components varies as the angle of incidence. At vertical incidence the reflection coefficient for shear waves is near zero, increasing to a maximum in the range of 30.degree.-45.degree.. P-waves on the other hand are not so affected. Those data can be used to generate pseudo-shear wave seismic sections. Thus, although Goins uses a P-wave source and shear-wave amplitude data derived therefrom, he does not use converted shear waves explicitly.
Garotta and Granger, in a paper entitled Acquisition and processing of 3C.times.3D data using converted waves, published in Expanded Abstracts for the 58th Annual SEG International meeting, 1988, pp 995-997, employ a 2-component, transverse and radial receiver rotation and an angle-dependent scaling algorithm followed by cross correlation between the data that propagate in orthogonal directions to determine the optimum angles of rotation. The method becomes unstable when the transverse component is small. The method does not explicitly combine energy polarized in different directions at common conversion points and it is therefore only sensitive to the principal near-surface shear wave coordinates.
Mallick and Frazer in a paper entitled Reflection/transmission coefficients and azimuthal anisotropy in marine seismic studies. published in the Geophysical Journal Int, (1991) v. 105, pp 241-252, teach a method wherein a two-component ocean bottom seismometer (OBS) is emplaced on the sea floor. A surface air gun is fired at spaced-apart intervals along two separate lines of survey at right angles to each other to generate converted P to S reflections at acoustic interfaces and water-bottom converted shear wave signal components x.sub..perp., x.sub..parallel., y.sub..perp., y.sub..parallel.. They then apply an Alford rotation operator (to be explained below) to the signal components to define the azimuthal direction of birefringence, the strike of a fracture plane for example. The method applies the same rotation angle for both receiver and source orientations and therefore is only sensitive to the principal directions of anisotropy at the water bottom.
In the real world, the alignment of a line of survey with respect to a fracture plane or the principle anisotropic axis of a formation is not known a priori. R. M. Alford, in SEG Expanded Abstracts, 56th Annual Meeting of the Society of Exploration Geophysicists, pp 476-479, which is incorporated herein by reference, teaches mathematical rotation operators which may be applied to multi-component, multi-source PS shear-wave data to align the observation coordinate frame with the natural coordinate system of the earth. As developed by Alford, a 4-component rotation is applied to vertically propagating S-waves in a 1-D earth at a given common midpoint. Those components consist of in-line S-wave source signals, detected by in-line and cross-line receivers and cross-line source signals detected by the same two in-line and cross-line receivers. This provides a 2.times.2 matrix of 4-component S-wave data as a function of time. The two diagonal elements of the matrix are the in-line S-wave source signals detected by the in-line horizontal receiver and the cross-line source signals detected by the cross-line horizontal receiver. The off-diagonal elements of the 2.times.2 matrix are the in-line S-wave source signals detected by the cross-line receiver and the cross-line S-wave source signals detected by the in-line horizontal receiver. The orthogonally-polarized sources and receivers are mathematically iteratively rotated by angular increments. Upon convergence, when aligned with the preferred earth coordinate system, the diagonal components of the matrix will contain the principle S-wave energy and the off-diagonal elements will contain little or no coherent S-wave energy. The teachings of that paper are confined to symmetrical SS shear wave field data and only provide polarization information of the near-surface layers.
The object of shear wave studies is to learn something about the principle polarization axis of a target formation. In the presence of anisotropic material above the target formation, the perceived surface axial rotation will be distorted; it will reflect the axial alignment of the uppermost anisotropic layer. Donald F. Winterstein, in U.S. Pat. No. 5,060,203, issued Oct. 22, 1991, teaches a method for stripping off the influence of upper layers to predict the subsurface stress regimes. Winterstein analyzes and removes the polarization changes of split S-waves as a function of depth using the direct downgoing wavefield in 4-component S-wave vertical seismic profile (VSP) data. His approach involves an application of the Alford rotation operators to minimize the energy on the off-diagonal components at the shallow-most level. The observed time shift of the two principal S-waves on the diagonal components is removed by time-shifting the slow S-wave to align with the fast S-wave. That time shift corresponds to removing the source delay and so is applied to two of the four seismic components and effectively removes the azimuthal anisotropy thus making the material between the source and the shallow medium isotropic. The process may be applied to additional layers as needed. The process is primarily applicable to VSP operations using shear waves wherein a unidirectional trajectory exists between a surface source and a down-hole receiver (or the reverse by reason of the principle of reciprocity). He does not teach application of the method to converted shear waves gathered along conventional surface line-surveys or 3-D operations.
An important purpose of shear-wave seismic surveys is to determine the strike of vertical fracture planes of a subsurface, target earth formation. In the oil country, horizontal boreholes are drilled, deviated to extend perpendicularly to the strike of the fracture planes to thereby maximize hydrocarbon fluid recovery.
There is a need for an economical robust method for measuring the orientation of the principle axis of a target formation, lying beneath a birefringent medium, using a P-wave source for generating converted shear waves.